(1) Field of the Invention
The present invention relates to an optical transmission system and an optical transmission method for transmitting a signal light while amplifying the signal light by utilizing Raman amplification, and more particularly, to a technique for controlling the Raman amplification so as to suppress deterioration of transmission characteristics due to the nonlinear optical effect.
(2) Related Art
For example, in a wavelength-division multiplexing (WDM) optical transmission system for multiplexing a plurality of optical signals of mutually different wavelengths to transmit these multiplexed optical signals through a single optical fiber transmission path, it is important to densely multiplex optical signals of wavelengths (wavelength channels) as many as possible, so as to increase a transmission capacity of the optical transmission system. However, there is a possibility that, as a result of densely multiplexing the wavelength channels, the nonlinear optical effect, such as four wave mixing or cross phase modulation, occurs in the optical fiber transmission path, so that the inter-channel interference is caused by an influence of the nonlinear optical effect, to thereby deteriorate the transmission characteristics. In order to avoid such deterioration of transmission characteristics, it is effective to reduce an input light power to an optical fiber, to thereby suppress the occurrence of nonlinear optical effect. However, this brings a reduction of optical signal-to-noise ratio (OSNR), thereby causing a possibility of deteriorating the transmission characteristics.
As conventional technique to suppress the aforementioned deterioration of transmission characteristics, there has been proposed a technique, as disclosed in Japanese Unexamined Patent Publication No. 2000-330145, in which, in an optical repeater provided with a typical optical amplifier, a pumping light is supplied to an optical fiber transmission path that is connected to the optical repeater, to perform distributed Raman amplification, so that an output light power from the optical repeater, i.e., an input light power to an optical fiber transmission path at the latter repeating section, is reduced to thereby decrease the influence of nonlinear optical effect. Further, in Japanese Unexamined Patent Publication No. 2000-299522, there has been proposed an optical transmission system in which the distributed Raman amplification using an optical fiber transmission path as an amplifying medium is combined with discrete Raman amplification in an amplifying medium within an optical repeater. Such a system using both of the distributed Raman amplification and discrete Raman amplification is considered to be more effective to suppress deterioration of transmission characteristics due to the nonlinear optical effect to be caused in the optical fiber transmission path.
For optical transmission systems utilizing the Raman amplification, in addition to the above techniques, various techniques have been proposed. Japanese Unexamined Patent Publication No. 10-73852 discloses an optical repeating transmission system provided with optical components for Raman amplification within an optical repeater so as to compensate for a loss in transmission path. Further, Japanese Unexamined Patent Publication No. 10-22931 discloses an optical amplifying transmission system that Raman amplifies a signal light being propagated through a transmission path, by pumping lights of a plurality of wavelengths, to make a signal band to be a broad band. Moreover, Japanese Unexamined Patent Publication No. 2000-314902 discloses a distributed Raman amplifier in which a power difference is caused between a pumping light at a shorter wavelength side and a pumping light at a longer wavelength side, in order to solve the wavelength dependence of the OSNR. Additionally, Japanese Unexamined Patent Publication No. 2001-109025 has proposed a method for reducing a gain fluctuation in a Raman amplifier and for reducing a gain variation for each gain medium fiber.
Furthermore, Japanese Unexamined Patent Publication No. 11-84440 discloses an optical transmission system capable of providing a flat and wide gain band and of compensating for dispersion of transmission path, by combining a Raman amplifying section with a rare earth element doped fiber amplifying section. Further, Japanese Unexamined Patent Publication No. 2001-15845 discloses a technique to automatically control an output level to be constant in a case of performing the distributed Raman amplification.
Meanwhile, in the aforementioned optical transmission system that uses both of the distributed Raman amplification and discrete Raman amplification, it is effective to utilize, as an amplifying medium for causing the discrete Raman amplification within the optical repeater, a medium such as a dispersion compensation fiber having a larger non-linearity and a higher Raman amplifying efficiency, compared with a typical optical fiber transmission path to be connected to an optical repeater. In a case where the discrete Raman amplification is to be used using the dispersion compensation fiber, in order to effectively suppress the transmission characteristic deterioration due to the nonlinear optical effect caused in the entire system, it is necessary to design the system, taking account of not only the influence of nonlinear optical effect caused in the optical fiber transmission path and the like, but also the influence of nonlinear optical effect caused in the dispersion compensation fiber and the like within the optical repeater.
In the aforementioned prior art, although the transmission characteristic deterioration due to the nonlinear optical effect caused in the optical fiber transmission path is reducible, there is not considered the influence of nonlinear optical effect caused in the optical repeater within which the discrete Raman amplification is conducted. Therefore, there is a problem that it is difficult to reliably suppress the transmission characteristic deterioration to be caused in the entire system or each repeating section as a unit.
Concerning techniques for suppressing the nonlinear optical effect caused in an optical fiber transmission path, in Japanese Unexamined Patent Publication No. 10-200509, there is disclosed an optical transmission system that sets a gain per unit length of a distributed amplifying medium on an optical fiber transmission path, so that the maximum intensity of WDM signal light is less than a defined value for causing the nonlinear optical effect. Although this optical transmission system is not applied with the discrete Raman amplification in an optical repeater, the controlling method to be set here for suppressing the nonlinear optical effect considers only the nonlinear optical effect caused in the optical fiber transmission path. Thus, even by this system, it is also difficult to effectively suppress the nonlinear optical effect caused in the entire system or each repeating section.
There will be described in detail hereinafter the influence of nonlinear optical effect in an optical transmission system using both of the distributed Raman amplification and discrete Raman amplification, while showing a specific calculation example.
FIG. 22 is a block diagram showing an example of a WDM optical transmission system using both of the distributed Raman amplification and discrete Raman amplification.
In the WDM optical transmission system of FIG. 22, for example, optical signals of different wavelengths generated at a plurality of optical senders (OSs) are wavelength multiplexed by a multiplexer, and transmitted to an optical fiber transmission path comprising a single mode fiber (SMF), to be sent towards optical receivers (ORs). Here, as the WDM signal light to be transmitted, it is assumed to include optical signals of 40 waves (xcex1 to xcex40) arranged in a C-band (for example, a wavelength band between 1529 nm and 1561 nm) at intervals of 100 GHz. A plurality of optical repeaters are arranged on the optical fiber transmission path at intervals of 100 km, while each optical fiber transmission path between adjacent optical repeaters is supplied with a 1.45 xcexcm band pumping light that is propagated in a direction opposite to the WDM signal light. Each optical repeater is constituted of a discrete Raman amplifier using a dispersion compensation fiber (DCF) as an amplifying medium and an erbium doped fiber amplifier (EDFA) combined with each other. For the SMF to be used for the optical fiber transmission path, there are supposed transmission parameters: a loss to the signal wavelength (1.55 xcexcm band) being 0.25 dB/km; a loss to the excitation wavelength (1.45 xcexcm band) being 0.50 dB/km; an effective area being 80 xcexcm2; a nonlinear refractive index being 2.9xc3x9710xe2x88x9220 m2/W; a dispersion coefficient to the signal wavelength being 17 ps/nm/km; and a dispersion slope being 0.057 ps/nm2/km, so that each chromatic dispersion caused in the optical fiber transmission path of each repeating section is compensated for by 100% by means of the DCF within each optical repeater.
Concerning the case where the above system model and parameters are assumed, FIG. 23 shows a calculation result of an OSNR within one repeating section and a nonlinear phase shift amount caused in the repeating section relative to a pumping light power for the discrete Raman amplification within each optical repeater. Note, the signal light power in the optical fiber transmission path and the noise light power accompanying the Raman amplification are obtained by numerically solving relational equations described in an article: H. Kidorf et al., xe2x80x9cPump interactions in a 100-nm bandwidth Raman amplifierxe2x80x9d, IEEE Photonics Technol. Lett., 11, 530-532 (1999). Further, the nonlinear phase shift amount is calculated, in accordance with the relationship represented by the following equation (1):                               Δφ          NL                =                                            2              ⁢                              xe2x80x83                            ⁢              π                        λ                    ·                      ∫                                                                                                      n                      2                                        ⁡                                          (                      z                      )                                                        ·                                      P                    ⁡                                          (                      z                      )                                                                                                            A                    eff                                    ⁡                                      (                    z                    )                                                              ⁢                              ⅆ                z                                                                        (        1        )            
wherein xcex94xcfx86NL represents a nonlinear phase shift amount; n2(z) represents a nonlinear refractive index; Aeff(z) represents an effective area; and P(z) represents an optical power at a position xe2x80x9czxe2x80x9d.
As shown in the calculation result of FIG. 23, it can be understood that, although it becomes possible to obtain an excellent OSNR by increasing the pumping light power for the discrete Raman amplification, the nonlinear phase shift amount is also increased. The nonlinear phase shift amount can be regarded as a parameter corresponding to the amount of nonlinear optical effect caused in one repeating section. Thus, it is assumed that, with the increase of this parameter, the influence of a self phase modulation (SPM) and a cross phase modulation (XPM) or the like is increased, thereby causing waveform degradation to thereby bring deterioration of transmission characteristics.
The present invention has been achieved in view of the above problems, and it is therefore an object of the present invention to provide a controlling technique for improving transmission characteristics in an optical transmission system and an optical transmission method using both of the distributed Raman amplification and discrete Raman amplification, while taking account of nonlinear optical effect to be caused in each repeating section including not only an optical fiber transmission path but also a discrete Raman amplifying medium within each optical transmission device.
To achieve the above object, the present invention provides an optical transmission system utilizing Raman amplification, which comprises a distributed Raman amplifying section that supplies a pumping light to an optical fiber transmission path to Raman amplify a signal light being propagated through the optical fiber transmission path, and a discrete Raman amplifying section that supplies a pumping light to an amplifying medium within an optical transmission device connected to the optical fiber transmission path to Raman amplify a signal light being propagated through the amplifying medium, wherein the optical transmission system further comprises a controlling device that controls the supplying conditions of the pumping lights at the distributed Raman amplifying section and the discrete Raman amplifying section, based on signal light powers at a signal light input point to the optical fiber transmission path and a signal light output point from the optical fiber transmission path, and signal light powers at a signal light input point to the amplifying medium within the optical transmission device and a signal light output point from the amplifying medium within the optical transmission device.
According to the optical transmission system having such a constitution, the supplying conditions of the pumping lights for the distributed Raman amplification and discrete Raman amplification are controlled, based on the signal light powers at the signal light input/output points of the optical fiber transmission path on which the distributed Raman amplification is performed and the signal light powers at the signal light input/output points of the amplifying medium of the optical transmission device in which the discrete Raman amplification is performed, thereby enabling to control the Raman amplification taking account of an influence of nonlinear optical effect caused not only in the optical fiber transmission path but also in the amplifying medium within the optical transmission device.
As a specific constitution of the optical transmission system, the controlling device may comprise a signal light power detecting section that detects signal light powers at the respective points, and a controlling section that controls the powers or wavelengths of the pumping lights to be supplied by the distributed Raman amplifying section and the discrete Raman amplifying section, so that the signal light powers at the respective points detected by the signal light power detecting section approach control target values of signal light powers for the respective points, respectively, which bring an amount of nonlinear optical effect caused in a transmission section including the optical fiber transmission path and the amplifying medium within the optical transmission device to a previously set value or less.
According to the optical transmission system having such a constitution, the signal light powers at the respective points are controlled so as to suppress the amount of nonlinear optical effect caused in the transmission section including the optical fiber transmission path and the amplifying medium within the optical transmission device to a required value or less. Thus, it becomes possible to avoid an increase of nonlinear optical effect accompanying the Raman amplification, to thereby obtain an excellent OSNR.
In the optical transmission system, the optical transmission device may include an optical amplifying section that amplifies the signal light Raman amplified by the discrete Raman amplifying section, and the controlling device may also control an amplifying operation of the optical amplifying section, based on the signal light powers at the respective points. This enables an increased output of the optical transmission device.
Further, in the optical transmission system, the distributed Raman amplifying section may multiplex a plurality of pumping lights of different wavelengths, to supply the multiplexed pumping light to the optical fiber transmission path, the discrete Raman amplifying section may multiplex a plurality of pumping lights of different wavelengths, to supply the multiplexed pumping light to the amplifying medium within the optical transmission device, and the controlling device may control the powers or wavelengths of the pumping lights, so that a gain wavelength characteristic of the Raman amplification by the discrete Raman amplifying section approaches a characteristic reverse to a gain wavelength characteristic of the Raman amplification by the distributed Raman amplifying section.
According to such a constitution, it becomes possible to flatten a Raman gain in one transmission section, by the combination of the distributed Raman amplifying section and discrete Raman amplifying section.
The present invention further provides an optical transmission method utilizing the Raman amplification for supplying a pumping light to an optical fiber transmission path to perform the distributed Raman amplification on a signal light being propagated through the optical fiber transmission path, and for supplying a pumping light to an amplifying medium within an optical transmission device connected to the optical fiber transmission path to perform the discrete Raman amplification on a signal light being propagated through the amplifying medium within the optical transmission device, wherein the supplying conditions of the pumping lights for the distributed Raman amplification and for the discrete Raman amplification are controlled, based on signal light powers at a signal light input point to the optical fiber transmission path and a signal light output point from the optical fiber transmission path, and signal light powers at a signal light input point to the amplifying medium within the optical transmission device and a signal light output point from the amplifying medium within the optical transmission device.
Further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.